Not given in the exam, recall it.
What it is for
Electric charge is used in Electricity (SPM Physics Form 5). Keep the SI units in every line of working. Electricity
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| Q | electric charge | C |
| I | current | A |
| t | time | s |
Rearrangements
I = Q / tt = Q / I
Worked example
Common trap
Understanding electric charge
Q is the electric charge in coulombs (C), I is the current in amperes (A), and t is the time for which the current flows in seconds (s). Current is the rate of flow of charge, so the total charge that passes a point equals the current multiplied by the time: Q = It.
One coulomb is the charge carried when a current of one ampere flows for one second (1 C = 1 A s).
Because the three quantities are linked by Q = It, you can find the current with I = Q / t or the time with t = Q / I. Always convert the time to seconds before substituting; a time given in minutes or hours must be changed first, or the charge value will be wrong.
A worked example, step by step
A current I = 0.50 A flows through a torch bulb for t = 4.0 minutes. Find the charge that passes through the bulb.
- Convert the time to seconds: t = 4.0 min = 4.0 × 60 = 240 s.
- Write the formula: Q = It.
- Substitute with units: Q = 0.50 A × 240 s = 120 C.
The answer is Q = 120 C, to two significant figures. The unit check confirms it: A × s = A s = C. If you had forgotten to convert minutes to seconds and used t = 4.0, you would get 2.0 C, sixty times too small, which shows why the time conversion matters.
Common mistakes and how this is tested
The most frequent error is leaving the time in minutes or hours instead of seconds; charge in coulombs needs time in seconds. Another is confusing current (A) with charge (C), current is the rate of flow, charge is the total amount.
- Mixing up Q = It with other electrical formulas such as V = IR; Q = It links only charge, current and time.
- Forgetting that the ampere itself means one coulomb per second.
Questions ask you to calculate the charge that flows in a given time, define the coulomb in terms of ampere and second, or explain how current relates to the flow of charge. State clearly that one coulomb is one ampere-second, and show the time conversion line for the method mark.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)